The Difference between "the ..." and "the only ..."
Because an article "the" means somebody-something definite is one from many, I wonder if there is a difference between "the (person, word, city, etc.)" and "the only (person, word, city, etc.)"? Example: "Among my friends, the only peson who can play the piano is Pete." vs. "Among my friends the person who can play the piano is Pete." If I've used "the" here it already means that there is no any another guy who can play the piano, Pete is "the only". I think since there is the definite article "the" in English which already contains the meaning "one and only", why do we need to use the word "only"? Just to emphasize the fact that this essence is the only? Doesn't "the only" sound like a tautology for native English speakers?

Thank you.

Question

The Difference between "the ..." and "the only ..."
Because an article "the" means somebody-something definite is one from many, I wonder if there is a difference between "the (person, word, city, etc.)" and "the only (person, word, city, etc.)"? Example: "Among my friends, the only peson who can play the piano is Pete." vs. "Among my friends the person who can play the piano is Pete." If I've used "the" here it already means that there is no any another guy who can play the piano, Pete is "the only". I think since there is the definite article "the" in English which already contains the meaning "one and only", why do we need to use the word "only"? Just to emphasize the fact  that this essence is the only? Doesn't "the only" sound like a tautology for native English speakers?

Thank you.

Answer

You are quite right. It is used purely for emphasis, and is quite natural and common, when used correctly.
Of course you don't need to use it - you can choose to if you like.

Answer

In that particular context you are right. The word "only" is superfluous. It is not incorrect though. It is mainly to help avoid confusion.

Answer

Adding the word "only" is a good tactic to check if your choice of "the" as the article needed is correct. The usage of articles at all may seem emphatic to speakers of languages without articles. But they don't feel emphatic to us.